WASHINGTON — President Donald Trump on Sunday promised a “big price” to be paid for what he said was a chemical weapons attack that choked dozens of Syrians to death the day before, and a top White House official said the administration would not rule out a missile strike to retaliate against the government of President Bashar Assad.

In a tweet, Trump laid the blame for the attack partly on President Vladimir Putin of Russia, the first time since his election that he has criticized the Russian leader by name on Twitter. Putin’s forces have been fighting for years to keep the Assad government in power amid Syria’s brutal civil war.

Trump also left no doubt that he believed the assessment of aid groups that Assad’s military had used chemical weapons to inflict the carnage Saturday in Douma, a rebel-held suburb of Damascus. The attack left at least 42 people dead in their homes from apparent suffocation and sent many others to clinics with burning eyes and breathing problems.

“Many dead, including women and children, in mindless CHEMICAL attack in Syria,” Trump wrote. “Area of atrocity is in lockdown and encircled by Syrian Army, making it completely inaccessible to outside world. President Putin, Russia and Iran are responsible for backing Animal Assad. Big price to pay. Open area immediately for medical help and verification. Another humanitarian disaster for no reason whatsoever. SICK!”

Thomas Bossert, Trump’s Homeland Security adviser, said he and the rest of the president’s national security team had been in talks with Trump late Saturday and early Sunday about how to respond. Asked specifically about the possibility of a missile strike, Bossert did not rule it out.

“I wouldn’t take anything off the table,” Bossert said on ABC’s “This Week.” “These are horrible photos; we’re looking into the attack at this point.”

Sen. Lindsey Graham, R-S.C., said Trump should make good on what the president appeared to be threatening on Twitter.

If the president “doesn’t follow through and live up to that tweet, he’s going to look weak in the eyes of Russia and Iran,” Graham said on “This Week.” “This is a defining moment.”

“You need to follow through with that tweet,” he added. “Show a resolve that Obama never did to get this right.”